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An experiment is rolling two fair dice. a) What is the probability that the sum is 8 ? Round answer to 3 decimal places. \( \square \) b) What is the probability that the sum is 8 given that the first die is a 3 ? Round answer to 3 decimal places. \( \square \) c) What is the probability that the first die is a 3 given that the sum is 8 ? Round answer to 3 decimal places. \( \square \) d) Are the events the sum is 8 and the first die is a 3 independent events? Why or why not? Round answer to 3 decimal places. Select an answer e) What is the probability that the sum is 7 ? Round answer to 3 decimal places. \( \square \) f) What is the probability that the sum is 7 given that the first die is a 2? Round answer to 3 decimal places. \( \square \) g) What is the probability that the first die is a 2 given that the sum is \( 7 ?^{\circ} \) Round answer to 3 decimal places. \( \square \) h) Are the events the sum is 7 and the first die is a 2 independent events?

Ask by Collins Guerrero. in the United States
Mar 10,2025

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a) The probability that the sum is 8 is 0.139. b) The probability that the sum is 8 given that the first die is 3 is 0.167. c) The probability that the first die is 3 given that the sum is 8 is 0.200. d) The events the sum is 8 and the first die is 3 are not independent because \( P(\text{sum}=8 \text{ and first die}=3) \neq P(\text{sum}=8) \times P(\text{first die}=3) \). e) The probability that the sum is 7 is 0.167. f) The probability that the sum is 7 given that the first die is 2 is 0.167. g) The probability that the first die is 2 given that the sum is 7 is 0.167. h) The events the sum is 7 and the first die is 2 are independent.

Solución

a) There are 36 equally likely outcomes when rolling two dice. The outcomes with a sum of 8 are \[ (2,6),\ (3,5),\ (4,4),\ (5,3),\ (6,2) \] There are 5 such outcomes. Thus, the probability is \[ P(\text{sum} = 8) = \frac{5}{36} \approx 0.139. \] b) Given that the first die is a 3, the sample space reduces to the outcomes \[ (3,1),\ (3,2),\ (3,3),\ (3,4),\ (3,5),\ (3,6) \] There are 6 outcomes. The only outcome where the sum is 8 is \((3,5)\) because \(3+5=8\). Thus, \[ P(\text{sum}=8 \mid \text{first die}=3) = \frac{1}{6} \approx 0.167. \] c) When the sum is 8, the possible outcomes are \[ (2,6),\ (3,5),\ (4,4),\ (5,3),\ (6,2) \] There are 5 outcomes. Only one of these, \((3,5)\), has the first die equal to 3. Hence, \[ P(\text{first die}=3 \mid \text{sum}=8) = \frac{1}{5} = 0.200. \] d) Two events \(A\) (the sum is 8) and \(B\) (the first die is 3) are independent if \[ P(A \text{ and } B) = P(A) \times P(B). \] We have \[ P(A) = \frac{5}{36} \quad \text{and} \quad P(B) = \frac{6}{36} = \frac{1}{6}. \] Thus, \[ P(A) \times P(B) = \frac{5}{36} \times \frac{1}{6} = \frac{5}{216} \approx 0.023. \] But previously, we found \[ P(A \text{ and } B) = P((3,5)) = \frac{1}{36} \approx 0.028. \] Since \(\frac{1}{36} \neq \frac{5}{216}\), the events are not independent. e) The outcomes with a sum of 7 are \[ (1,6),\ (2,5),\ (3,4),\ (4,3),\ (5,2),\ (6,1) \] There are 6 outcomes. Hence, \[ P(\text{sum}=7) = \frac{6}{36} = \frac{1}{6} \approx 0.167. \] f) Given that the first die is 2, the possible outcomes are \[ (2,1),\ (2,2),\ (2,3),\ (2,4),\ (2,5),\ (2,6) \] For the sum to be 7, we need \(2+{\text{second die}}=7\) so that the second die must be 5. There is 1 outcome. Therefore, \[ P(\text{sum}=7 \mid \text{first die}=2) = \frac{1}{6} \approx 0.167. \] g) When the sum is 7, the outcomes are \[ (1,6),\ (2,5),\ (3,4),\ (4,3),\ (5,2),\ (6,1) \] Out of these 6 outcomes, only one has the first die equal to 2: \((2,5)\). Thus, \[ P(\text{first die}=2 \mid \text{sum}=7) = \frac{1}{6} \approx 0.167. \] h) To check if the events "the sum is 7" and "the first die is 2" are independent, we verify whether \[ P(\text{sum}=7 \text{ and first die}=2) = P(\text{sum}=7) \times P(\text{first die}=2). \] We have \[ P(\text{sum}=7) = \frac{6}{36} = \frac{1}{6} \quad \text{and} \quad P(\text{first die}=2) = \frac{6}{36} = \frac{1}{6}. \] Thus, \[ P(\text{sum}=7) \times P(\text{first die}=2) = \frac{1}{6} \times \frac{1}{6} = \frac{1}{36}. \] Since the event "sum is 7 and first die is 2" corresponds exactly to the outcome \((2,5)\), \[ P(\text{sum}=7 \text{ and first die}=2) = \frac{1}{36}. \] Because both sides are equal, the events are independent.

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Bonus Knowledge

To find the probability that the sum of two dice is 8, we first list all possible pairs of dice that yield this sum: (2,6), (3,5), (4,4), (5,3), and (6,2). That's 5 successful outcomes out of 36 possible combinations (since each die has 6 sides). Thus, the probability is \( \frac{5}{36} \approx 0.139 \), rounded to three decimal places. For part b, if the first die is a 3, the possible sums that give us 8 can only come from the second die being a 5. So, there is only 1 successful outcome (3,5) out of 6 total possibilities (since the second die can still be any number from 1 to 6). The probability here is \( \frac{1}{6} \approx 0.167 \), rounded to three decimal places. For part c, if we look at the sum being 8, the successful outcomes are the same as before. However, only one of those outcomes (3,5) has the first die as a 3. The total successful outcomes for the sum being 8 is still 5. Therefore, the conditional probability is calculated as \( \frac{1}{5} \approx 0.200 \), rounded to three decimal places. In part d, we check independence. Events are independent if the occurrence of one does not affect the occurrence of another. Here, knowing the first die is 3 does influence the overall sum needing to be 8 (there’s only one successful outcome). Hence, they are not independent. Moving to part e, for the sum to equal 7, the successful pairs are (1,6), (2,5), (3,4), (4,3), (5,2), and (6,1)—that's 6 outcomes. Therefore, the probability is \( \frac{6}{36} = \frac{1}{6} \approx 0.167 \), rounded to three decimal places. For part f, if the first die is a 2, then the only scenario leading to a sum of 7 is the second die being a 5, making that just a single successful outcome. Therefore, the probability is \( \frac{1}{6} \approx 0.167 \), rounded to three decimal places. Next, for part g, the successful outcomes for the sum being 7 are 6, and only 2 of those outcomes (2,5, and 5,2) involve the first die being a 2. So, the probability is \( \frac{1}{3} \approx 0.333 \), rounded to three decimal places. Lastly, part h checks the independence of the sum being 7 and the first die showing 2. Similar to part d, the outcome of having the sum of 7 does depend on the first die’s result. Therefore, these events are not independent.

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